A MTJ memory element is also referred to as a MTJ nanopillar or MTJ cell and is a key component in magnetic recording devices, and in memory devices such as magnetoresistive random access memory (MRAM) and spin torque transfer (STT)-MRAM. An important step in fabricating an array of MTJ cells (MTJs) is patterning a photoresist layer on an uppermost hard mask in a MTJ stack of layers. The pattern is produced with a photolithography process, and is etch transferred into the hard mask which then serves as an etch mask during a second etch that transfers the pattern through the MTJ stack to form an array of MTJs each with a critical dimension (CD) that in state of the art devices is substantially less than 100 nm from a top-down view. The first etch may be a reactive ion etch (RIE) while the second etch involves either RIE or an ion beam etch (IBE).
A MTJ stack of layers includes two ferromagnetic layers called the free layer (FL) and reference layer (RL), and a dielectric layer (tunnel barrier) between the FL and RL. The RL has a fixed magnetization preferably in a perpendicular-to-plane direction (perpendicular magnetic anisotropy or PMA) while the FL is free to rotate to a direction that is parallel or anti-parallel to the RL magnetization direction thereby establishing a “0” or “1” memory state for the MTJ. The magnetoresistive ratio is expressed by dR/R where dR is the difference in resistance between the two magnetic states when a current is passed through the MTJ, and R is the minimum resistance value.
Precise patterning is required to generate uniform island features in the photoresist layer that are subsequently processed to form non-interacting MTJ devices each having a CD that is within a tight tolerance in order to ensure substantially uniform magnetic properties from one MTJ device to the next. Furthermore, etch transfer processing through the MTJ stack of layers is challenging since there are a variety of materials (magnetic alloys, non-magnetic metals, and dielectric films) that each have a different etch rate when subjected to IBE with Ar, or to fluorocarbon or oxygen based RIE. Thus, the hard mask must have sufficient thickness and etch resistance to remain intact during the MTJ etch.
As CDs decrease significantly below 100 nm in advanced memory products, processing becomes increasingly more expensive in order to meet performance requirements in terms of minimum CD, and CD uniformity across the wafer. For example, the exposure wavelength of incident light in the photolithography process used to pattern the photoresist layer may be reduced from 248 nm (KrF source) to 193 nm (ArF source) to print features with smaller CDs, and higher yields. However, ArF exposure tools are significantly more expensive than KrF exposure tools. Moreover, process latitude that includes depth of focus, and exposure latitude also becomes more difficult to control as the CD target for the photolithography step approaches 60 nm or lower. Optical proximity correction (OPC) of chrome patterns on quartz masks that are used during photolithography exposures is often relied upon to assist in generating the desired photoresist feature shapes and sizes but adds design complexity and cost. Thus, processing steps that can shrink the CD after the initial pattern is formed by photolithography, and that avoid relying on more expensive exposure tools and OPC in quartz masks to deliver a minimum CD less than 60 nm, are highly desirable in minimizing manufacturing cost.
Another important consideration is to maintain a sufficient thickness for the photoresist pattern while generating the minimum CD so that the photoresist mask survives a subsequent etch through the underlying hard mask. The photoresist cannot be too thick or the aspect ratio (thickness/width) of the resulting features will be too large and prevent small CDs from being printed. Accordingly, photoresist thickness is preferably maintained during any post photolithography process that decreases the CD before the etch transfer through the hard mask.